The invention relates to a method for manufacturing multi-walled pipes and the multi-walled pipe, manufactured according to the method.
The manufacturing of the pipes specified above is fundamentally known. A steel sheet is coated on one or both sides with a solder layer of copper. The steel sheet forms a steel source layer and the copper forms one or two solder source layers of a coated metal sheet. The coated metal sheet, or strips of the sheet, are subsequently rolled up multiple times. In a final step, the copper is melted by heating the multi-walled pipe, by means of which the rolled up walls of the pipe become soldered and thus a high quality, material-bonded connection is obtained. Pipes manufactured in this manner are very robust on one hand, and relatively economical to manufacture on the other hand.
A substantial problem with this method concerns the coating of the steel sheet with the solder layer, for example. This is because, for a large-scale coating, as in the case of steel sheets, electroplating (referred to as galvanic technology) is used predominantly. A sufficient adhesion of the solder layer to steel is achieved from an economical perspective, however, solely and exclusively through the addition of cyanide to the electrolyte baths for the electroplating. A number of approaches have existed for improving the adhesion of the solder layer in order to avoid the addition of the extremely poisonous cyanide to the electrolyte baths. Every one of these approaches, however, is accompanied by disadvantages, such that in the overall assessment, the known method of adding cyanide has prevailed so far. These disadvantages are, for example, an insufficient adhesion, a too high susceptibility of the electrolyte baths to contaminants, the addition of dangerous strong oxidants, or process steps that are far too complex. All of these disadvantages result in either a poor quality of the pipe (in particular the poor adhesion of the solder layer) or uneconomical methods.